U.S. patent application number 10/820463 was filed with the patent office on 2004-11-18 for calibration scheme for logarithmic image sensor.
This patent application is currently assigned to STMicroelectronics Ltd.. Invention is credited to Hurwitz, Jonathan Ephriam David, Storm, Graeme.
Application Number | 20040227831 10/820463 |
Document ID | / |
Family ID | 32981958 |
Filed Date | 2004-11-18 |
United States Patent
Application |
20040227831 |
Kind Code |
A1 |
Storm, Graeme ; et
al. |
November 18, 2004 |
Calibration scheme for logarithmic image sensor
Abstract
A logarithmic pixel is formed by a photodiode connected to a
semiconductor device that is operating based upon a sub-threshold.
A logarithmic output is taken from an output node connected to the
pixel via an amplifier. To calibrate the pixel, the photodiode is
isolated by a switch and a ramp voltage is applied as reference
voltage to the amplifier. The ramp voltage acts across the constant
internal capacitance of the pixel to produce in-pixel a constant
current for calibration purposes.
Inventors: |
Storm, Graeme; (Edinburgh,
GB) ; Hurwitz, Jonathan Ephriam David; (Edinburgh,
GB) |
Correspondence
Address: |
ALLEN, DYER, DOPPELT, MILBRATH & GILCHRIST P.A.
1401 CITRUS CENTER 255 SOUTH ORANGE AVENUE
P.O. BOX 3791
ORLANDO
FL
32802-3791
US
|
Assignee: |
STMicroelectronics Ltd.
Marlow - Buckinghamshire
GB
|
Family ID: |
32981958 |
Appl. No.: |
10/820463 |
Filed: |
April 8, 2004 |
Current U.S.
Class: |
348/294 ;
348/E3.021 |
Current CPC
Class: |
H04N 5/35518
20130101 |
Class at
Publication: |
348/294 |
International
Class: |
H04N 005/335 |
Foreign Application Data
Date |
Code |
Application Number |
May 6, 2003 |
EP |
03252836.6 |
Claims
1-10. (Cancelled).
11. An image sensor comprising: an array of pixels, each pixel
comprising a photodiode, a semiconductor device having a
capacitance and being connected to said photodiode and operating
based upon a sub-threshold for providing a signal that is
proportional to a logarithm of light intensity on said photodiode,
and a calibration circuit having a capacitance and for applying a
voltage having a constant rate of change across the capacitance
associated with said semiconductor device and said calibration
circuit for producing a constant current within said pixel.
12. An image sensor according to claim 11, wherein each pixel
further comprises a switching device between said photodiode and
said semiconductor device, said switching device being operable
during calibration for isolating said photodiode from said
semiconductor device.
13. An image sensor according to claim 12, wherein said calibration
circuit comprises an amplifier having an inverting input for
receiving the signal from said semiconductor device, a
non-inverting input for receiving a reference voltage, and an
output for providing a pixel output signal.
14. An image sensor according to claim 13, wherein the reference
voltage comprises a ramp voltage for providing the voltage having
the constant rate of change.
15. An image sensor according to claim 14, wherein the ramp voltage
is also applied at a beginning of an image-capturing operation of
said pixel.
16. An image sensor according to claim 13, further comprising a
feedback loop between the output of said amplifier and said
semiconductor device, the feedback loop for controlling said
semiconductor device.
17. An image sensor according to claim 13, wherein each pixel has
an image area associated therewith, and said amplifier for each
respective pixel is completely within the corresponding image
area.
18. An image sensor according to claim 13, wherein each pixel has
an image area associated therewith, and wherein said amplifier for
each respective pixel is partly within the corresponding image
area.
19. An image sensor according to claim 13, wherein said
semiconductor device comprises a transistor comprising a conducting
terminal, and wherein the capacitance is provided by a capacitance
of the conducting terminal and a capacitance of the inverting input
of said amplifier.
20. An image sensor comprising: an array of pixels, each pixel
comprising a photodiode; a semiconductor device having a
capacitance and being connected to said photodiode; and a
calibration circuit having a capacitance and for applying a voltage
across the capacitance associated with said semiconductor device
and said calibration circuit for producing a constant current
within said pixel.
21. An image sensor according to claim 20, wherein the image sensor
is operating in a logarithmic mode.
22. An image sensor according to claim 20, wherein each pixel
further comprises a switching device between said photodiode and
said semiconductor device, said switching device being operable
during calibration for isolating said photodiode from said
semiconductor device.
23. An image sensor according to claim 20, wherein said calibration
circuit comprises an amplifier having an inverting input for
receiving the signal from said semiconductor device, a
non-inverting input for receiving a reference voltage, and an
output for providing a pixel output signal.
24. An image sensor according to claim 23, wherein the reference
voltage comprises a ramp voltage for providing the voltage having
the constant rate of change.
25. An image sensor according to claim 24, wherein the ramp voltage
is also applied at a beginning of an image-capturing operation of
said pixel.
26. An image sensor according to claim 23, further comprising a
feedback loop between the output of said amplifier and said
semiconductor device, the feedback loop for controlling said
semiconductor device.
27. An image sensor according to claim 23, wherein each pixel has
an image area associated therewith, and said amplifier for each
respective pixel is completely within the corresponding image
area.
28. An image sensor according to claim 23, wherein each pixel has
an image area associated therewith, and wherein said amplifier for
each respective pixel is partly within the corresponding image
area.
29. An image sensor according to claim 23, wherein said
semiconductor device comprises a transistor comprising a conducting
terminal, and wherein the capacitance is provided by a capacitance
of the conducting terminal and a capacitance of the inverting input
of said amplifier.
30. A method for calibrating an image sensor operating in a
logarithmic mode, the image sensor comprising an array of pixels,
each pixel comprising a photodiode, a semiconductor device having a
capacitance and connected to the photodiode, and a calibration
circuit having a capacitance and being connected to the
semiconductor device, the method comprising: applying a voltage
having a constant rate of change across the capacitance associated
with the semiconductor device and the calibration circuit for
producing a constant current within the pixel during
calibration.
31. A method according to claim 30, wherein each pixel further
comprises a switching device between the photodiode and the
semiconductor device; the method further comprising operating the
switching device during calibration for isolating the photodiode
from the semiconductor device.
32. A method according to claim 31, wherein the semiconductor
device operates based upon a sub-threshold for providing a signal
that is proportional to a logarithm of light intensity on the
photodiode, and the calibration circuit comprises an amplifier
having an inverting input for receiving the signal from the
semiconductor device, a non-inverting input for receiving a
reference voltage, and an output of the amplifier provides a pixel
output signal.
33. A method according to claim 32, wherein the reference voltage
comprises a ramp voltage for providing the voltage having the
constant rate of change.
34. A method according to claim 33, wherein the ramp voltage is
also applied as the reference voltage at a beginning of an
image-capturing operation of the pixel.
35. A method according to claim 32, wherein each pixel further
comprises a feedback loop between the output of the amplifier and
the semiconductor device, the feedback loop for controlling the
semiconductor device.
36. A method according to claim 32, wherein each pixel has an image
area associated therewith, and wherein the amplifier for each
respective pixel is contained completely within the corresponding
image area.
37. A method according to claim 32, wherein each pixel has an image
area associated therewith, and wherein the amplifier for each
respective pixel is partly within the corresponding image area.
38. A method according to claim 32, wherein the semiconductor
device comprises a transistor comprising a conducting terminal, and
wherein the capacitance is provided by a capacitance of the
conducting terminal and a capacitance of the inverting input of the
amplifier.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to electronics, and in
particular, to a solid-state image sensor.
BACKGROUND OF THE INVENTION
[0002] Dynamic range is a very important parameter of any imaging
system. Human vision has the capability to see details across a
wide illumination range in a single scene, and is reported to
exhibit around 200 dB of dynamic range. Scenes in excess of 100 dB
are not uncommon in everyday situations. Consequently, designers of
CMOS and CCD image sensors are continuously looking for ways to
increase dynamic range.
[0003] Sensors having logarithmic characteristics have been used to
image scenes of high dynamic range. In a logarithmic mode the pixel
voltage is continuously available and no integration time is used.
In a typical CMOS arrangement, the induced photocurrent flows
through one or more MOS transistors and sets up a gate-source
voltage that is proportional to the logarithm of the photocurrent.
This is shown in FIG. 1 where the gate-source voltage appears
across the device M2. Since the photocurrent is very small, the MOS
device(s) will operate in a sub-threshold, and the voltage varies
logarithmically with the photocurrent. The voltage is read out by
source follower circuitry. Around six decades of light can be
captured in the logarithmic mode.
[0004] Due to the small size of the devices used in the pixels, a
high degree of mismatch results from process variations, and
produces fixed pattern noise (FPN) across the array. Logarithmic
sensors cannot use double sampling (in its conventional form) for
mismatch removal since this technique only removes the variation of
the device M1 and does not alter the effect of device M2. This
arises from the fact that the logarithmic architecture operates
continuously in time and has no reference state.
[0005] Another disadvantage of the logarithmic arrangement is a
slow response time for low light levels. Increased photocurrent for
a given light level can be accomplished by increasing the size of
the light sensing element, but this is not desirable since the cost
for a given resolution will increase accordingly.
[0006] Calibrating the pixels addresses the FPN problem, that is,
by bringing them into a reference state so that the FPN can be
learned and then cancelled. The common way to calibrate a
logarithmic pixel on-chip is to pull a matched current through the
load device of each pixel using a current source in each column.
This places the pixel into a known reference state that should be
equivalent to illuminating the sensor with a uniform intensity.
However, this requires an extra vertical line in each column for
the current source, and the associated capacitance of the extra
line prevents small calibration currents from settling quickly.
[0007] U.S. Pat. No. 6,355,965 to He et al. shows an arrangement in
which a calibration access transistor shorts the source follower,
and the calibration is performed without the need for an extra
vertical line. But this still has problems of a long settling time
for low photocurrents.
[0008] Kavadias discloses in the article "A Logarithmic Response
CMOS Image Sensor With On-Chip Calibration", IEEE Journal of
solid-state circuits, vol. 35, No. 8, August 2000, a high
calibration current being pulled through the load device. This
disclosure uses an NMOS transistor and capacitor in a column
instead of a constant current source. The calibration point can
therefore be far from the operating point of the pixel due to the
difference between photo and calibration currents.
[0009] Loose et al. discloses in the article "A Self-Calibrating
Single-Chip CMOS Camera with Logarithmic Response", IEEE Journal of
solid-state circuits, vol. 36, No. 4, April 2001, a correction
voltage being stored in an analog memory (a capacitor) in the pixel
such that the signal voltage is free from offsets. The entire
amplifier is in the column, and an extra vertical line is used to
access the current source.
SUMMARY OF THE INVENTION
[0010] The invention provides an image sensor as defined in claim
1, and a method of calibrating an image sensor as defined in claim
10. Preferred features and advantages of the invention will be
apparent from the other claims and from the following
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] An embodiment of the invention will now be described, by way
of example only, with reference to the drawings, in which:
[0012] FIG. 1 is a schematic diagram of a pixel in an image sensor
according to the prior art; and
[0013] FIG. 2 is a schematic diagram of a pixel in an image sensor
forming one example of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] Referring to FIG. 2, a pixel has a photodiode P which causes
a photocurrent to flow through device M2, thus causing it to
operate in a sub-threshold (assuming device M5 is on). The
photodiode voltage at the node pix is used as the inverting input
to amplifier A. The non-inverting input receives a reference
voltage Vref. The node pix will be held at the reference voltage
Vref (plus the offset of the amplifier) and the logarithmic result
will be available at the output of the amplifier A.
[0015] To calibrate the pixel, the node pix is isolated from the
photodiode P by device M5 to eliminate the effects of the
photocurrent. The reference voltage Vref is now ramped, and due to
the amplifier feedback loop, the voltage at the node pix will try
to track it. As the pixel voltage rises it will induce a current
which must be supplied through M2.
[0016] The ramp voltage is applied to make use of the fact that a
constant current can be generated if there is a constant voltage
change across a constant capacitance. Using the formula I=C*dV/dt
and knowing the capacitance, a voltage ramp can be programmed to
produce a constant current.
[0017] In FIG. 2, the capacitance of the pixel is given by the
capacitance on the drain of device M5 and the gate capacitance of
the inverting input of the amplifier A. If these capacitances are
well matched across the array then the calibration currents will be
matched.
[0018] This arrangement allows the pixels to be calibrated without
the use of additional vertical lines and current sources. It also
allows very small calibration currents to be produced without the
settling time problems associated with current sources and vertical
access lines with large capacitance.
[0019] As well as providing a calibration current, the ramping of
the reference voltage Vref can be used to aid the settling time of
the circuit when the device M5 is on and the logarithmic voltage is
dependent on the photocurrent. The node pix is able to charge up
quickly as the feedback loop will cause the device M2 to turn on
more quickly and supply more current. However, the node pix can
only discharge with the current supplied from the photodiode P
which could be very small and cause a very long settling time. If
the amplifier has any overshoot then this settling time could be a
problem for low photocurrents. By ramping the reference voltage
Vref the oscillations can be absorbed by the ramp and at the end of
the ramping period the circuit should settle more quickly.
[0020] The amplifier can be completely within the pixel, as shown.
Alternatively, the amplifier could be formed partly within the
pixel and partly within the column and switched between pixels as
required. The invention thus provides an improvement in calibrating
a logarithmic pixel.
* * * * *